Method of Aiding Uplink Beamforming Transmission

ABSTRACT

A method of aiding uplink transmission is disclosed. One method includes a base station downlink transmitting signals to at least a first terminal, the transmitted signals directed and conveying data to at least the first terminal. The method further includes a second terminal eavesdropping the transmitted signals, and measuring a signal quality. The second terminal estimates uplink channel information based on the measured signal quality, for aiding uplink transmission. Based at least in part on the estimated transmission channel, the second terminal transmits uplink signals to the base station.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/074,186, filed Feb. 29, 2008, which claims the benefit of U.S.Provisional Application No. 61/062,629, filed Jan. 28, 2008, which areincorporated by reference in their entirety herein.

FIELD OF THE DESCRIBED EMBODIMENTS

The described embodiments relate generally to wireless communications.More particularly, the described embodiments relate to a method andsystem of aiding uplink beamforming transmission.

BACKGROUND

Mobile WiMAX is a wireless platform being developed to provide wirelessaccess that is able to deliver on demanding service requirements. Withthe added support for a variety of advanced multi-antennaimplementations, Mobile WiMAX offers wireless access that meets growingnetwork demands with higher performance, fewer sites, less spectrum, andreduced cost.

Multiple antenna techniques at the base station and end-user device,paired with sophisticated signal processing, can dramatically improvethe communications link for the most demanding application scenariosincluding heavily obstructed propagation environments and high speedmobility service. Where conventional wireless network design has longused base site sectorization and single, omni-directional antennas atthe end-user device to serve the communications link, with advancedmulti-antenna implementations operators have a new techniques to developthe robust wireless networks.

Industry vendors and sources have created a host of naming conventionsto refer to multi antenna implementations. Simply put, the term MIMO(multiple input multiple output) can be used to reference anymulti-antenna technologies. MIMO antenna systems are used in codedivision multiple access (CDMA) networks, time division multiplexing(TDM) networks, time division multiple access (TDMA) networks,orthogonal frequency division multiplexing (OFDM) networks, orthogonalfrequency division multiple access (OFDMA) networks, and others. Inorder to maximize throughput, MIMO networks use a variety of channelestimation techniques to measure the transmission channel between a basestations of the wireless network and a mobile device. The channelestimation technique used depends on the wireless network type (i.e.,CDMA, TDM/TDMA, OFDM/OFDMA).

Methods of estimating the transmission channels rely on pilots thatoccupy valuable time and frequency transmission space. That is, thetypical methods require dedicated signaling overhead which requiresbandwidth (time and/or frequency), and therefore, reduces systemcapacity.

Additionally, wireless networks that include mobile terminals(subscribers) have transmission channels that change frequently.Therefore, the transmission channels must be re-estimated or updatedmore frequently. As a result, wireless networks that include mobileterminals require even more capacity dedicated to channel estimations.

It is desirable to have a system and method for estimating and/orupdating transmission channel information between a base station and amobile terminal of a wireless network that can reduce the amount of timeand frequency channel capacity required for the pilots.

SUMMARY

An embodiment includes a method of aiding uplink beamformingtransmission. The method includes a base station downlink transmittingbeamformed signals to at least a first terminal, the beam formed signalsdirected and conveying data to at least the first terminal. The methodfurther includes a second terminal eavesdropping the transmittedbeamformed signals, and measuring a signal quality. The second terminalestimates uplink channel information based on the measured signalquality, for aiding uplink transmission. Based at least in part on theestimated transmission channel, the second terminal transmits beamformeduplink signals to the base station.

Another embodiment includes another method of aiding uplinktransmission. The method includes a base station downlink transmittingbeamformed signals to a plurality of terminals. The beam formed signalsare directed and convey data to each of the plurality of terminals. Eachbeamformed signal directed to each of the plurality of terminals arespatially orthogonal to the other beamformed signals directed to otherterminals. The method further includes a second terminal eavesdroppingthe transmitted beamformed signals, and measuring a signal quality. Thesecond terminal estimates uplink channel information based on themeasured signal quality, for aiding uplink transmission. The secondterminal uplink transmits beamformed uplink signals to the base station,wherein the beamformed uplink signals are formed at least in part basedon the estimated uplink channel information.

Another embodiment includes another method of aiding uplinktransmission. The method includes a terminal receiving downlinktransmitted beamformed signals, and measuring a signal quality, whereinthe downlink transmitting beamformed signals are transmitted from a basestation to a plurality of terminals. The beam formed signals aredirected and convey data to each of the plurality of terminals, and eachbeamformed signal directed to each of the plurality of terminals isspatially orthogonal to the other beamformed signals directed to otherterminals. The method further includes the terminal estimating downlinkchannel information based on the measured signal quality. The terminaluplink transmits beamformed uplink signals to the base station, thebeamformed uplink signals formed at least in part based on the estimateduplink channel information.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a base station transmitting beamformedsignals to multiple user devices.

FIG. 2 shows an example of a base station transmitting beamformedsignals to multiple user devices, and an eavesdropping user receiving atleast some of the beamformed signals.

FIG. 3 shows an example of downlink and uplink frame that can be usedfor scheduled wireless communications.

FIG. 4 shows a flow chart that includes steps of one example of a methodof aiding uplink beamforming transmission.

FIG. 5 is a flow chart that includes steps of one other example of amethod.

FIG. 6 is a flow chart that includes steps of another example of amethod of aiding uplink transmission.

FIG. 7 is a flow chart that includes steps of another example of amethod of aiding uplink transmission.

DETAILED DESCRIPTION

Methods and apparatuses for updating transmission channel informationbased on eavesdropping of beamformed signals are disclosed. Terminals(subscribers) of a wireless network eaves-drop beam formed signalsdirected from a base station to other terminals. The eavesdroppingterminals measure at least one signal quality of the eavesdroppedsignals. Based upon the measured signal qualities, estimates of atransmission channel between the base station and the eaves droppingterminal can be generated and/or updated. Additionally, for some systems(for example, a time division duplex (TDD) wireless system) theeavesdropping terminal can select uplink sounding conditions based onthe measured signal qualities.

It is to be understood that for the descriptions of the embodiments, theterm “eavesdropping” is not intended to be a derogatory term. A terminalthat is eavesdropping, is receiving data and/or pilot signals that aredirected (generally, through beamforming) to a different terminal. Inthe described embodiments, an eavesdropping terminal acts benignly andis not necessarily interested in the decoding the scheduled dataintended for another terminal. The eavesdropping terminal instead takesadvantage of a situation that includes a base station transmittingsignals to other terminals, to learn, obtain, or estimate informationabout a transmission channel between the base station and itself. Thisinformation can be fed back to the base station, enabling the basestation to improve channel estimations to the eavesdropping terminal.This information can alternatively or additionally be used by theeavesdropping terminal to make or improve uplink channel estimationsbetween the eavesdropping terminal and the base station. The intentionis that the eavesdropping terminal is scheduled at another time for datatransmission or reception based on the channel information obtainedduring eavesdropping. At that point, the original terminals receivingdata can become eavesdroppers, and so on.

FIG. 1 shows an example of a base station 110 transmitting beamformedsignals to multiple user devices 120, 122, 124. The base station 110 anduser devices 120, 122, 124 can be a part of a wireless network, forexample, a WiMax wireless network. To capitalize on the performanceadvantages offered by MIMO wireless systems, the wireless network caninclude MIMO communication diversity, MIMO spatial multiplexing and/orbeamforming.

MIMO communication diversity includes a single data stream beingreplicated and transmitted over multiple antennas. For example, theredundant data streams can each be encoded using a mathematicalalgorithm known as Space Time Block Codes. With this example of coding,each transmitted signal is orthogonal to the rest reducingself-interference and improving the capability of the receiver todistinguish between the multiple signals. With the multipletransmissions of the ceded data stream, there is increased opportunityfor the receiver to identify a strong signal that is less adverselyaffected by the physical path. The receiver additionally can use forexample, Maximal-Ratio Combining (MRC) techniques to combine themultiple signals for more robust reception, MIMO communication diversityis fundamentally used to enhance system coverage.

MIMO spatial multiplexing includes the signal to be transmitted beingsplit into multiple data streams and each data stream is transmittedfrom a different base station transmit antenna operating in the sametime-frequency resource allocated for the receiver. In the presence of amultipath environment, the multiple signals arrive at the receiverantenna array with sufficiently different spatial signatures allowingthe receiver to readily discern the multiple data streams. Spatialmultiplexing provides a very capable means for increasing the channelcapacity.

Wireless networks can also include adaptive mode selection between MIMOcommunication diversity and MIMO spatial multiplexing. In environmentswhere the Signal to Noise Ratio (SNR) is low, such as the edge of thecell or where the signal is weak, MIMO communication diversity mayoutperform MIMO spatial multiplexing. At higher SNR, where the system ismore prone to be bandwidth limited rather than signal strength limited,MIMO spatial multiplexing may outperform MIMO communication diversity.An ideal WiMAX system employing MIMO techniques supports both. Thesystem calculates an optimal switching point and dynamically shiftbetween the two approaches to offer the necessary coverage or capacitygains demanded from the network at any given time or location.

Wireless network systems, such as, WiMAX systems, can also usebeamforming as a means to further increase system coverage and capacitycan surpass the capabilities of MIMO techniques. Beamforming techniquessuch as Statistical Eigen Beamforming (EBF) and Maximum RatioTransmission (MRT) are optional features in the 802.16e WiMAX standard,but some vendors are taking advantage of its strong performancecharacteristics.

Beamforming techniques leverage arrays of transmit and receive antennasto control the directionality and shape of the radiation pattern. Theantenna elements have spatial separation dictated by the wavelength oftransmission and are supported by signal processing.

Channel information can be communicated from the WiMAX subscriber to theWiMAX base station using the uplink sounding response. Based on theunderstanding of the channel characteristics, the WiMAX base stationutilizes signal processing techniques to calculate weights to beassigned to each transmitter controlling the phase and relativeamplitude of the signals. By leveraging constructive and destructiveinterference, the radiation pattern is steered and formed to provide anoptimal radiation pattern focused in the direction of communication.

When transmitting a signal, beamforming can increase the power in thedirection the signal is to be sent. When receiving a signal, beamformingcan increase the receiver sensitivity in the direction of the wantedsignals and decrease the sensitivity in the direction of interferenceand noise.

Beamforming techniques allow the WiMAX system to realize increased rangewith higher antenna gain in the desired direction of communications andbetter connectivity between the base station and device. Simultaneously,the narrower beamwidth and reduced interference increases the capacityand throughput offered by the system.

Estimated transmission channel information is needed for properselection between beamforming and/or spatial multiplexing transmission.As previously described, the channel information fez mobile networkschanges more rapidly than for static networks, and therefore, thechannel information needs to be updated more frequently.

In OFDM-MIMO systems, training sequence design as well as efficientchannel estimation algorithm remains a challenge if different trainingsequence, signals are transmitted from different antennassimultaneously. Several approaches based on training tones have beenattempted in prior art systems. Other known solutions of a timeorthogonal pre-amble scheme can be implemented, but typically increasethe overhead.

FIG. 2 shows an example of a base station 210 transmitting beamformedsignals to multiple user devices 220, 222, 224, and a second terminal226 receiving at least some of the beamformed signals. For this example,the beamformed signals are intended for the user devices 220, 222, 224.That is, the beamformed signals are directed to the multiple userdevices 220, 222, 224, but not directed to the second terminal 226. Foran embodiment, each beamformed signal for each user device is formed tobe orthogonal to the beamformed signals for the other devices.

As shown, the second terminal 226 eaves-drops the beamformed signals (asshown by arrows 250, 252) that arc directed to the other user devices220, 222, 224. The base station 210 uses channel information between thebase station 210 and the user devices 220, 222, 224 to form the beams.Based on knowledge of signal quality of signals eavesdropped by thesecond terminal 226, the base station 210 can obtain some informationabout a transmission channel between the base station 210 and the secondterminal 226. The base station 210 can obtain the signal quality fromuplink transmission (shown by arrow 260) from the second terminal 226.That is, the second terminal measures the signal quality of thebeamformed signals and feeds the signal quality measurements back to thebase station 210. The base station can use this information to estimatethe channel between the base station 210 and the second terminal 226, orsupplement prior estimates of the channel between the base station 210and the second terminal 226.

The base station 210 transmits beamformed signals to multiple userdevices. That is, the base station 210 transmits multiple access signalsthat are received by multiple terminals, and that can be defined by atleast frequency and time slots. As will be described, the multiple beamformed signals allocated to different users can also be spatiallyorthogonal, and/or include cyclic delay diversity. The signal qualitymeasurements of the eavesdropped signals need to have an identifier thatallows the base station 210 to identify which beamformed signalstransmitted by the base station 210 are being eavesdropped, and resultedin the signal quality measurement.

One method of transmitting wireless signal, such as wirelesstransmission according to the WiMax protocol, includes multi-carriersymbols (such as OFDM) organized according to downlink and uplinkframes. The frames include frequency carriers and time slots that can beused to identify the beamformed signals.

FIG. 3 shows an example of downlink and uplink frame that can be usedfor scheduled wireless communications. The downlink frame and the uplinkframe include sub-carriers (1024 sub-carriers are shown in FIG. 3) andtime slots for OFDM symbols (33+15=48 symbols are shown in FIG. 3). Thedownlink frame includes a preamble and a MAP. A particular carrierfrequency and a particular symbol can used to identify a tile within thedownlink and/or uplink frames. The downlink includes pilot tones thatare typically used to characterize a downlink channel between a basestation and mobile terminal.

The uplink frame includes uplink sounding symbols (also referred to aspilot sub-carriers) that can be used to characterize an uplink channelbetween the mobile terminal and the base station. Guards TTG 310 and RTG320 are included between the downlink and uplink frames.

The MAP includes a schedule of the downlink and uplink transmission.Based on the MAP, the mobile terminals can identify tile in which dataand pilots are located. The MAP includes the previously discussedidentifying information.

FIG. 4 shows a flow chart that includes steps of one example of a methodof aiding uplink beamforming transmission. A first step 410 includes abase station downlink transmitting beamformed signals to at least afirst terminal, the beam formed signals directed and conveying data toat least the first terminal. A second step 420 includes a secondterminal eavesdropping the transmitted beamformed signals, and measuringa signal quality. The eave-dropped beamformed signals are not directedto the second terminal. A third step 430 includes the second terminalestimating uplink channel information based on the measured signalquality, for aiding uplink transmission.

The uplink channel information is used by the second terminal to aiduplink transmission to the base station. More specifically, the secondterminal transmits beamformed signals to the base station, wherein thebeamformed uplink signals are formed at least in part based on theuplink channel information.

Based on the estimated transmission channel information, the secondterminal creates or updates channel information between the secondterminal and the base station based on the measured signal quality, andthe transmission identifier. That is, if the second terminal does notpresently have information regarding the transmission channel betweenthe second terminal and the base station, the second terminal can createthe transmission channel information based on the measured signalquality. If the second terminal does have information regarding thetransmission channel, then the second terminal can update the existingchannel information based on the measured signal quality.

The beamformed signals can be data or pilot signals. That is, both datasignals and pilot signals can be transmitted as beamformed signals.Therefore, it is possible for the eavesdropping terminal to measure thesignal quality of, and identify both data and pilot signals.

As described, the base station can transmit beam formed signals tomultiple terminals. Therefore, the second terminal can receive thetransmitted beamformed signals intended for multiple differentterminals. The transmitted beamformed signal directed to one or more ofthe multiple terminals, but are not directed to the second terminal. Thesecond (eavesdropping) terminal can measure corresponding signalqualities, and identify which other terminal the beamformed signals weredirected to.

The second terminal estimates the uplink channel based at least in parton reception (eavesdropping) of downlink transmitted signals. Therefore,the second terminal establishes a relationship between an uplink channelassociated with the uplink channel information, and a correspondingdownlink channel. This relationship can generally be established if thecommunication between the base station and the second terminal is timedomain duplex (TDD) transmission. For TDD transmission, the uplink anddownlink channels can be assumed to be approximately the same assumingthe channel to be reciprocal. The downlink transmission of the basestation and uplink transmission of the second terminal occur atdifferent times tux TDD transmission.

The beamformed signals eavesdropped by the second terminal can beintended for multiple other terminals. Additionally, each beamformedsignal directed to each of the plurality of terminals can be spatiallyorthogonal to the other beamformed signals directed to other terminals.

For one embodiment, the second terminal identifies beamformed signalsthat have a quality above a threshold. For another embodiment, thesecond terminal identifies beamformed signals having a quality below athreshold. The beamformed signals can be identified as described.

An embodiment includes the second (eavesdropping) terminal measuringsignal qualities of eavesdropped signals, but also identifying channelsfor transmitting uplink sounding based on the measured signal quality,of the beamformed signals.

For MIMO transmission, the second (eavesdropping) terminal can includemultiple antennas, and include the second terminal receiving thetransmitted beamformed signals over multiple receive antennas, andmeasuring a joint signal quality indicator. The joint signal qualityindicator can include measured signal qualities of multiple beams at themultiple receive antennas. Based on the joint signal quality indicator,the second terminal can determine whether the second terminal is totransmit in the uplink in a beamforming mode or a spatial multiplexingmode based on the joint signal quality indicator.

For an embodiment, the joint signal quality indicator includes a signalquality difference between the multiple receive antennas. if the signalquality difference between the multiple antennas varies by greater thana threshold, then the second terminal selects spatial multiplexing fortransmission to the base station. If the measured signal quality betweenthe multiple receive antennas varies by less than a threshold, then thesecond terminal selects beamforming for transmission to the basestation.

An embodiment includes the base station cyclic-delaying the transmittedbeamformed signals. This embodiment additionally includes the secondterminal selecting frequency sub-carriers of the cyclic-delayedtransmitted beamformed signals for reception and signal qualitymeasurement, based on a frequency dependency of the frequencysub-carriers of the cyclic-delayed transmitted beamformed signals.

FIG. 5 shows a frequency spectrum of a downlink multi-carrier signal inwhich different sub-carriers of the multi-carrier signal are allocatedto different terminals, and the downlink multi-carrier signal includescyclic delay diversity. As shown, a first sub-carrier 510 is allocatedto a first user U0, a second sub-carrier 512 is allocated to a seconduser U1, a third sub-carrier 514 is allocated to a third user U2 and afourth sub-carrier is allocated to a fourth user U3.

An embodiment includes beamforming each sub-carrier of a multi-carrierOFDMA symbol directed to the terminals. The terminals may be assignedrandomly or in specific patterns to subcarriers. Generally, M transmitantennas of the base station can support K terminals with spatiallyorthogonal beams q₁, . . . k_(K), in which K≦M. A terminal k can beassigned to subcarrier k with beam q_(k) for k=1, . . . K. Because thesubcarriers in an OFDMA system are generally closely-spaced, it can beassumed that an eavesdropper has a frequency-flat channel response fromthe M basestation antennas to its receive antennas. This means that thechannel does not vary significantly across the K subcarriers.

Assume in this example that the terminal has N receive antennas. Thenthe signal received by the eavesdropper on frequency k becomes:

y(k)=P·Gq _(k) ·d _(k) +n(k)

where:

G represents the N×M frequency-flat channel between the basestation andthe eavesdropper,

q_(k) represents the beam sent to terminal k, with ∥q_(k)∥=1 (unit-normbeam)

d_(k) represents the data or pilot signal sent to terminal k,

n(k) is additive receiver noise at the eavesdropper,

P represents the transmission power of the basestation across allantennas

Arranging the K measurements taken by the eavesdropper into an N×Kmatrix yields:

Y=GQ+n

where Q=[q₁ . . . q_(K)] is the M×K matrix of all the K terminal beams.This gives the eavesdropper an estimate of the product of G and Q. In aTDD system, the uplink channel of the eavesdropper is G^(T) where thesuperscript “T” denotes transpose. Since the eavesdropper has anestimate of GQ [or equivalently (GQ)^(T)] it may form the singular valuedecomposition

(GQ)^(T) =TRW*

where T is a K×K unitary matrix, R is a K×N diagonal matrix withpositive diagonal values, and W is an N×N unitary matrix, and where thesuperscript “*” denotes conjugate-transpose.

The eavesdropper then uses its N transmit antennas with the columns of Was beamforming vectors on the uplink. The basestation knows Q and canperform optimum processing of this received beamformed signal.

This embodiment is most effective if Q is a unitary (also sometimescalled “orthogonal’) matrix, meaning that the constituent beams areorthogonal to one another.

One possible way for the basestation to achieve orthogonality includesthe basestation arranging the channel information for the intendedbeamformed K terminals into a matrix:

$H = \begin{bmatrix}h_{1}^{*} \\\vdots \\h_{K}^{*}\end{bmatrix}$

where h*₁, . . . h*_(K) represent the 1×M channels between thebasestatton and the K terminals. The matrix H therefore has dimensionK×M. Rather than using the standard well-known unit-energy beamsq_(k)=h_(k)/∥k_(k)∥ (which are not guaranteed to be orthogonal), thebasestation forms the singular value decomposition:

H=USV*

where U is a K×K unitary matrix, S is a K×K diagonal matrix withpositive diagonal entries, and V* is an K×M matrix such that V*V=I (theidentity matrix). The basestation then forms:

Q=VU*

and chooses the K columns of Q as the unit-energy beams, one for each ofthe K terminals.

For the description here, Q can be referred to as an orthogonal set, andits columns can be used to generate spatially orthogonal beams. Anembodiment includes the base station forming orthogonal beamformedsignals by estimating a channel matrix, computing a singular valuedecomposition of the estimated channel matrix, computing an orthogonalset from the singular value decomposition, and generating spatiallyorthogonal beams by selecting columns of the orthogonal set.

Another way for a basestation to achieve orthogonality includes thebasestation transmitting a combination of beamforming and cyclic-delaydiversity, where the transmitted beam to the kth terminal is written as

${q_{k}(f)} = \begin{bmatrix}q_{1\; k} \\{q_{2\; k}^{2{\pi j\tau}_{2}f}} \\\vdots \\{q_{Mk}^{2{\pi j\tau}_{M}f}}\end{bmatrix}$

This beam has frequency-dependence given by the cyclic-delay parametersτ₂, . . . τ_(M), which generally have units of seconds. (adopting theconvention that τ₁=0.) The basestation may choose |q_(1k)|=|q_(2k)|= . .. |q_(Mk)| and the eavesdropping terminal can then generally find f₁ andf₂ such that

q* _(k)(f ₁)q _(k)(f ₂)=0.

Therefore, the beams to terminal k are orthogonal at two differentfrequencies. With proper choice of the cyclic-delay parameters, theeavesdropper can find f₁, . . . f_(M) such that q_(k)(f₁), . . .q_(k)(f_(M)) are all orthogonal. Thus, the eavesdropper can use thebeams at these frequencies for channel estimation.

FIG. 6 is a flow chart that includes steps of another example of amethod of aiding uplink transmission. A first step 610 includes a basestation downlink transmitting beamformed signals to a plurality ofterminals, the beam formed signals directed and conveying data to eachof the plurality of terminals, each beamformed signal directed to eachof the plurality of terminals being spatially orthogonal to the otherbeamformed signals directed to other terminals. A second step 620includes a second terminal eavesdropping the transmitted beamformedsignals, and measuring a signal quality. A third step 630 includes thesecond terminal estimating uplink channel information based on themeasured signal quality, for aiding uplink transmission. A fourth step640 includes the second terminal uplink transmitting beamformed uplinksignals to the base station, the beamformed uplink signals formed atleast in part based on the estimated uplink channel information.

As previously described, an embodiment includes the base stationcyclic-delaying the transmitted beamformed signals. A specificembodiment additionally melt the second terminal selecting frequencysub-carriers of the cyclic-delayed transmitted beamformed signals forreception and signal quality measurement, based on a frequencydependency of the frequency sub-carriers of the cyclic-delayedtransmitted beamformed signals.

An embodiment includes the second (eavesdropping) terminal augmentingselection of sounding symbols for uplink sounding based on the uplinkchannel information. More specifically, and embodiment includes soundingsymbols being selected only on frequency carriers in which beamformedsignals can be received from the base station having a predeterminedsignal quality.

FIG. 7 is a flow chart that includes steps of another example of amethod of aiding uplink transmission. A first step 710 includes theterminal receiving downlink transmitted beamformed signals, andmeasuring a signal quality, wherein the downlink transmitting beamformedsignals are transmitted from a base station to a plurality of terminals,the beam formed signals directed and conveying data to each of theplurality of terminals, each beamformed signal directed to each of theplurality of terminals being spatially orthogonal to the otherbeamformed signals directed to other terminals. A second step 720includes the terminal estimating uplink channel information based on themeasured signal quality. A third step 730 includes the terminal uplinktransmitting beamformed uplink signals to the base station, thebeamformed uplink signals formed at least in part based on the estimateduplink channel information.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific firthsor arrangements of parts so described and illustrated. The invention islimited only by the appended claims.

1. A method for aiding uplink transmission, comprising: eavesdropping,by a second terminal, on a signal transmitted from a base stationintended to a first terminal; measuring, by the second terminal, asignal quality of the eavesdropped signal; and estimating uplink channelinformation of the second terminal based on the measured signal qualityfor aiding uplink transmission.
 2. The method of claim 1, furthercomprising: transmitting a signal from the second terminal to the basestation, wherein the signal transmitted is formed at least in part basedon the estimated uplink channel information.
 3. The method of claim 1,further comprising: establishing a relationship between an uplinkchannel associated with the estimated uplink channel information and acorresponding downlink channel.
 4. The method of claim 1, wherein theuplink transmission from the second terminal to the base station and thedownlink transmission from the base station to the first terminal occurat different times.
 5. The method of claim 5, wherein the communicationbetween the second terminal and the base station is time domain duplex(TDD) transmission.
 6. The method of claim 1, wherein the signalcomprises a pilot signal.
 7. The method of claim 1, further comprising:the second terminal updating at least one signal that is formed to aidtransmission between the second terminal and the base station based onthe estimated uplink channel information.
 8. The method of claim 1,further comprising: receiving the signal by the second terminal overmultiple receive antennas.
 9. The method of claim 8, further comprising:measuring a joint signal quality indicator, wherein the joint signalquality indicator includes a signal quality difference between themultiple receive antennas.
 10. The method of claim 9, furthercomprising: the second terminal selecting beamforming for transmissionto the base station when the joint signal quality indicator varies byless than a threshold.
 11. The method of claim 9, further comprising:the second terminal selecting spatial multiplexing for transmission tothe base station when the joint signal quality indicator varies by morethan a threshold.
 12. A method for aiding uplink transmission,comprising: eavesdropping, by a second terminal, on a plurality ofsignals transmitted from a base station intended to a plurality of firstterminals, wherein each signal of the plurality is spatially orthogonalto other signals of the plurality; measuring, by the second terminal, asignal quality of the eavesdropped signals; and estimating uplinkchannel information of the second terminal based on the measured signalquality for aiding uplink transmission.
 13. The method of claim 12,further comprising: applying a cyclic-delay to the plurality of signalstransmitted to the plurality of first terminals.
 14. A system for aidinguplink transmission, comprising: a first terminal with an antenna; and asecond terminal with an antenna, wherein the second terminal is operableto eavesdrop on a signal intended to the first terminal transmitted froma base station, wherein the second terminal is further operable tomeasure the signal quality of the eavesdropped signal, and wherein thesecond terminal is further operable to estimate uplink channelinformation of the second terminal based on the measured signal quality.15. The system of claim 15, wherein the second terminal is furtheroperable to transmit a signal from the second terminal to the basestation, wherein the signal is formed at least in part based on theestimated uplink channel information.
 16. The system of claim 15,wherein the second terminal further comprises a second antenna.